Cogging torque in an inner rotor type permanent-magnet type synchronous motor comprises pulsating components in torque, which is generated between teeth of a stator core (stator iron core) and a magnet rotor (rotor) when the magnet rotor is rotated by an external drive at the time of non-current-carrying to a winding, and only an order of a least common multiple of the number 2p of magnetic poles of a rotor magnet and the number Z of teeth (slots) of a stator core appears theoretically (see Non-Patent Document 1). However, this theory is limited to the case where rotors (mainly, magnets) and stator cores are uniform in shape and material properties and manufactured completely symmetrically with respect to the number of poles and the number of slots.
Since the number of poles and the number of slots get out of symmetric property in real machines, in particular, in a field of volume production, however, components of cogging torque having lower orders than the order of the least common multiple appear in many cases at large amplitudes. An increase in cogging torque has a great influence upon performance of products because of causing degradation in positioning accuracy for servomotors and degradation in feeling of steering for motors for automotive power steering.
Returning to the principle of torque generation, an explanation will be given to a mechanism for generation of pulsating components in torque. Torque is related to magnetic flux density and increases in the case where magnetic flux is easy to pass. Easiness, with which magnetic flux passes, is called permeance (reciprocal of magnetic resistance), and torque is generated in proportion to the square thereof. Accordingly, when permeance is varied, cogging torque is generated. In the case where a magnet making a source of generation of magnetic flux involves a nonuniform distribution and a symmetric property inconsistent with the number of poles, these are sensed on a side of a stator and pulsation having orders consistent with the number of slots and higher order components thereof appears. Higher order components are composed of higher harmonic wave components, which nonuniform components do not necessarily make a near-sinusoidal wave change to thereby cause.
Since main magnetic flux passes through an air from a magnet to return to the magnet through a back yoke portion from teeth of a stator core, materials of passages are divided into two. One is an air which is present between a rotor and a stator, and the other is a magnetic body that makes a core (iron core). In recent years, a laminate of flat rolled magnetic steel sheets and strip is in many cases used for such magnetic body, the magnetic property of flat rolled magnetic steel sheets and strip causes a problem frequently. In the case where flow of main magnetic flux involves a nonuniform distribution and a symmetric property inconsistent with the number of slots, these are sensed on a side of a rotor and pulsation having orders consistent with the number of poles and higher order components thereof appears.
For passages of magnetic flux, magnetic permeability μ indicative of easiness, with which magnetic flux passes, is constant in an air, so that a quantity of magnetic flux in the air appears due to a change in length of gaps (air gaps). Physical quantities having an influence on main magnetic flux moving through a rotor magnet and stator teeth are roughly classified into two, one of which is a gap (called an air gap) indicative of a shortest distance between an outside diameter of the rotor magnet and the stator teeth, and the other of which is a gap (generally called an open width) between the adjacent stator teeth. While an air gap is determined by an outside diameter shape of a rotor and an inside diameter shape of a stator, the inside diameter shape of a stator causes a problem in many cases.
Also, in the case where in order to facilitate a winding process, a method, in which a core is partially or wholly divided between teeth, is employed instead of manufacturing a stator core from a substantially circular-shaped unitary core, minute clearances are present to make clearance gaps when the divided portions are joined together.
Also, in the case where a core is partially divided and joined after winding, for convenience of a joint process, such part is in some cases made different in structure from the remaining part whereby non-uniformity in structure is generated.
Subsequently, cores manufactured from a magnetic body such as flat rolled magnetic steel sheets and strip generate, in many cases, individual differences in magnetic permeability due to various factors and a nonuniform distribution in the same individual one. One of the factors for generation of individual differences is a composition (grade) of flat rolled magnetic steel sheets and strip being a core material. Also, one of the factors for generation of differences in the same individual one is due to those different magnetic permeabilities in specific portions of a core obtained by a method of punching a core shape, which are caused by a difference (called magnetic anisotropy) in magnetic property between a direction of rolling of, for example, flat rolled magnetic steel sheets and strip and a direction perpendicular thereto. Also, when flat rolled magnetic steel sheets and strip are punched by blades of a metallic die, forces exerted by the blades cause degradation of teeth end surfaces in magnetic permeability, and the process of fitting of concave and convex portions (called caulking) for fixation of a laminate causes degradation of a caulked portion and its neighborhood.
Further, the manufacturing process of mounting a frame on an outer periphery of a stator core to fix the same to a bracket that supports a bearing is in many cases performed in order to prevent a stator from being displaced by torque generated between a rotating rotor and the stator, but a force exerted on the outer periphery of the stator core by the frame has an influence on not only a neighborhood of the outer periphery of the stator core, through which magnetic flux does not pass so much, but also a neighborhood of teeth, which makes a main passage, to cause degradation of flat rolled magnetic steel sheets and strip, which makes a main passage of magnetic flux, in magnetic property and displacement of teeth, thus changing an inside diameter shape of the stator core.
Unless gaps and magnetic property are ideally uniformly formed for the number of poles and the number of slots, cogging torque of lower orders is generated.
As described above, cogging torque of orders consistent with the number Z of slots is generated due to non-uniformity on a side of a magnet, and cogging torque of orders consistent with the number 2p of poles is generated due to various factors, such as non-uniformity in air gap, non-uniformity in open width, non-uniformity in clearance gap, distribution of magnetic property with respect to magnetic anisotropy of flat rolled magnetic steel sheets and strip, distribution of magnetic property generated by partial degradation of magnetic permeability due to punching, caulking, and stress in a frame, nonuniform distribution of clearance gaps of a split core, structural non-uniformity of joints, etc.
These factors are inevitably generated in actual motors by a manufacturing method for enhancement in volume production, or a limit in working accuracy in manufacturing processes.
Trials for reduction in cogging torque have been made taking notice of such manufacturing processes. For example, in order to obtain uniformity in air gaps, JP-A-2001-218429 (see Patent Document 1) proposes measures of ensuring roundness of inside diameter by uniformly applying pressure over outer and inner peripheries of a core to fix the same when a stator is to be press fitted into a frame. In JP-A-09-23687 (see Patent Document 2), it is tried to reduce cogging torque due to magnetic anisotropy by displacing a direction of magnetic anisotropy from central angles of teeth.
Also, in JP-A-2001-95199 (see Patent Document 3), it is tried to prevent an increase in cogging torque by maintaining a frame as uniform as possible in thickness to maintain a force given to a stator by the frame uniformly and to prevent the stator from being nonuniformly changed in inside diameter shape. In JP-A-2001-258225 (see Patent Document 4) and JP-A-2002-272074 (see Patent Document 5), measures for restriction on the number of caulked portions have been proposed taking account of influences by caulking. Further, in JP-A-06-52346 (see Patent Document 6), lamination is made so as to arrange seams circumferentially at substantially equal intervals whereby it is tried to dissolve non-uniformity in magnetic flux, which is caused by the seams.
The inventors of the present application have researched components having the same orders as the number 2p of poles, among cogging torque having a smaller number of pulsation than a least common multiple of the number 2p of poles of a magnet and the number Z of slots of a stator and clarified that products manufactured in volume production appear in many cases as a result of superposition of cogging torque waveforms including an amplitude and a phase, of at least two or more factors. Accordingly, a fundamental understanding is obtained, in which measures taking notice of only one property as in the prior art are insufficient to enable adequately reducing cogging torque and measures for individual properties, for example, trying to make roundness approach zero cannot materialize a complete ideal state actually. In particular, for actual motors manufactured in volume production, it is difficult to unlimitedly make cogging torque approach zero without taking account of working accuracy. That is, a technique is demanded, in which cogging torque caused by working accuracy is grasped as net and cogging torque measured in a final stage of the manufacturing process is cancelled to be made zero.    Non-Patent Document 1: Daikoku, et. al, “Cogging Torque Examination in Permanent Magnet Motors with Stressed Stator Core,” Materials of workshop of rotating machinery of Electric Appliance Society RM-03-152, pp. 14-18 (November 2003)    Patent Document 1: JP-A-2001-218429    Patent Document 2: JP-A-09-23687    Patent Document 3: JP-A-2001-95199    Patent Document 4: JP-A-2001-258225    Patent Document 5: JP-A-2002-272074    Patent Document 6: JP-A-06-52346
In permanent-magnet type synchronous motors, cogging torque having the same orders as the number 2p of poles of a magnet is generated due to composite superposition of various factors, such as non-uniformity in air gap, non-uniformity in open width, non-uniformity in clearance gap, distribution of magnetic property with respect to magnetic anisotropy of flat rolled magnetic steel sheets and strip, distribution of magnetic property generated by partial degradation of magnetic permeability due to punching, caulking, and stress in a frame, nonuniform distribution of clearance gaps of a split core, structural non-uniformity of joints, etc. In this case, it is necessary to take notice of superposition of cogging torque waveforms including not only amplitudes but also phases, and it is necessary to take canceling measures to make cogging torque, which appears as a result of superposition, approach zero as consequence, in addition to taking compensating measures to separate and correctly estimate respective factors, in which amplitudes negate other properties and is apparently decreased, and to reduce individual amplitudes.
The invention has been thought of in order to solve the problems described above, and has its object to provide a permanent-magnet type synchronous motor, in which cogging torque having pulsating components of the same orders as the number 2p of poles of a magnet is decreased near to zero unlimitedly by separating composite individual factors of cogging torque and taking thorough measures of reducing the cause of large amplitudes in an experimental manufacturing stage, and regulating processes to superpose properties, in which phase control is possible, on properties, in which amplitudes cannot be decreased for the convenience of manufacture, to cancel the latter, and a method of manufacturing the same.